sbattach: fix --detach

sbattach --detach isn't working, as we're not properly setting sigbuf in
image_pecoff parse.

This change ensures we populate sigbuf when we find a valid cert table.
Also, add a test case for this.

Bug report & initial patch from from Steve Langasek.

Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>
3 files changed